Photoionization enhanced multiphoton

The general principle of detection of free radicals is based on the spectroscopy (absorption and emission) and mass spectrometry (ionization) or combination of both. An early review has summarized various techniques to detect small free radicals, particularly diatomic and triatomic species.68 Essentially, the spectroscopy of free radicals provides basic knowledge for the detection of radicals, and the spectroscopy of numerous free radicals has been well characterized (see recent reviews2-4). Two experimental techniques are most popular for spectroscopy studies and thus for detection of radicals laser-induced fluorescence (LIF) and resonance-enhanced multiphoton ionization (REMPI). In the photochemistry studies of free radicals, the intense, tunable and narrow-bandwidth lasers are essential for both the detection (via spectroscopy and photoionization) and the photodissociation of free radicals. 

In practice, for application to ambient air, efficient photoionization requires the use of pulsed lasers and multiphoton absorption methods. The terms multiphoton ionization, or MPI, and resonance-enhanced multiphoton ionization, or REMPI, are used to describe these processes. 

A number of techniques have been used previously for the study of state-selected ion-molecule reactions. In particular, the use of resonance-enhanced multiphoton ionization (REMPI) [21] and threshold photoelectron photoion coincidence (TPEPICO) [22] has allowed the detailed study of effects of vibrational state selection of ions on reaction cross sections. Neither of these methods, however, are intrinsically capable of complete selection of the rotational states of the molecular ions. The TPEPICO technique or related methods do not have sufficient electron energy resolution to achieve this, while REMPI methods are dependent on the selection rules for angular momentum transfer when a well-selected intermediate rotational state is ionized in the most favorable cases only a partial selection of a few ionic rotational states is achieved [23], There can also be problems in REMPI state-selective experiments with vibrational contamination, because the vibrational selectivity is dependent on a combination of energetic restrictions and Franck-Condon factors. 

The H2 molecule is a system for which quite recently it has been possible to measure in unprecedented detail state-selected vibrationally and rotation-ally resolved photoionization cross sections in the presence of autoionization [27-29]. The technique employed has been resonantly enhanced multiphoton ionization. The theoretical approach sketched above has been used to calculate these experiments from first principles [30], and it has thus been possible to give a purely theoretical account of a process involving a chemical transformation in a situation where a considerable number of bound levels is embedded in an ensemble of continua that are also coupled to one another. The agreement between experiment and theory is quite good, with regard to both the relative magnitudes of the partial cross sections and the spectral profiles, which are quite different depending on the final vibrational rotational state of the ion. 

The next stepping-stone to photoionization is finding the electronic levels of the neutral, because nonresonant ionization has rather low cross-sections that translate into poor ionization efficiencies along with high photon flux requirements. Resonant absorption of photons is more effective by several orders of magnitude [91]. Ideally, resonant absorption of the first photon leads to an intermediate state from where absorption of a second photon can forward the molecule into a continuum. This technique is known as 1 -i-1 resonance-enhanced multiphoton ionization (REMPI). From a practical point of view, the second photon should be, but not necessarily has to be, of the same wavelength (Fig. 2.20) [92]. Proper selection of the laser wavelengths provides compound-selective analysis at extremely low detection limits [90,91,93,94]. 

The mechanism of the multiphoton photoionization and fragmentation of polyatomic molecules by intense (>10 W/cm ) UV laser pulses was studied in detail by Boesl et al. (1980). The spectroscopic features of resonance-enhanced multiphoton excitation and ionization were reviewed by Ashfold and Howe (1994). 

However, this high variability in ionization efficiencies implies that LDl is a very selective ionization method. In some cases, this selectivity is advantageous, for example, when one wishes to observe the presence or concentration of one, known, select molecule of interest.However, in mass spectrometry, one usually is interested in detecting all molecules that are present, including unknown species. Thus, many methods were explored to ionize the molecules after they were desorbed by the laser. These methods are collectively known as laser desorption post-ionization methods, and the post-ionization techniques include electron impact (El, diagrammed in Figure 6.2), chemical ionization (Cl), photoionization (PI), resonant-enhanced multiphoton ionization (REMPI), and many others. 

Photoionization occurs via expulsion of an electron when a molecule absorbs the energy in one or more photons. Resonance-enhanced multiphoton ionization under supersonic jet conditions has great potential as a soft and highly selective ionization method. The resonance-enhanced process proceeds via the absorption of two photons in which the first photon excites the molecule to a resonant intermediate excited state, and the second photon ionizes the molecule. The ability to obtain energy-resolved ionization is a unique feature of resonance-enhanced photoionization. 

Multiphoton ionization MPI Photoionization Atomic and molecular ions Resonance-enhanced MPI is highly selective Trace analysis... 

By employing a laser for the photoionization (not to be confused with laser desorption/ ionization, where a laser is irradiating a surface, see Section 2.1.21) both sensitivity and selectivity are considerably enhanced. In 1970 the first mass spectrometric analysis of laser photoionized molecular species, namely H2, was performed [54]. Two years later selective two-step photoionization was used to ionize mbidium [55]. Multiphoton ionization mass spectrometry (MPI-MS) was demonstrated in the late 1970s [56—58]. The combination of tunable lasers and MS into a multidimensional analysis tool proved to be a very useful way to investigate excitation and dissociation processes, as well as to obtain mass spectrometric data [59-62]. Because of the pulsed nature of most MPI sources TOF analyzers are preferred, but in combination with continuous wave lasers quadrupole analyzers have been utilized [63]. MPI is performed on species already in the gas phase. The analyte delivery system depends on the application and can be, for example, a GC interface, thermal evaporation from a surface, secondary neutrals from a particle impact event (see Section 2.1.18), or molecular beams that are introduced through a spray interface. There is a multitude of different source geometries. 

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